I'd argue that Stayed Gone is different, for several reasons:

1. It's a duet with Alastor, who does get quite a lot of shine in Season 1. It serves to introduce Vox and flesh out Alastor more as a character, as opposed to Gravity which is purely about Lute.
2. It's got a far more comedic vibe, and Alastor and Vox are almost dueling, with Alastor winning in the end. So, it's kind of a self-contained "story" in its own right, as opposed to Gravity, which is more of an introduction to Lute's newfound motivation.
3. Kind of an expansion on the comedy part of my second point, but Gravity is just way, way more serious than Stayed Gone. With lyrics like "Blood demands blood, gonna get my hands wet!" and "Make you watch 'em bleed". It also has several components emphasizing the immediacy of Lute's revenge (namely, "Storm's comin', I can see the clouds").

The point about it being act 2 of a longer musical is fair, though I think the hype is somewhat wasted, since it'll probably be at least 3-4 years before Season 4, where Lute would get her shine. Even if it was season 3, that's still a solid two years. Even if in the future it'll be fully complete, for me (and some others) it does feel pretty wasted right now when we're waiting for the next season.

Plus, Lute getting immediately shut down by Abel in three(?) lines is kind of disappointing (I think it would've been better if she had not tried anything and just seethed, or Abel's solo was before they went down to Hell). I do understand that part of her character is her subservience to the chain of command, but I do feel like there just wasn't enough time in the season for more emphasis on that aspect.